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Share-based compensation - Changes in stock options (Details)
 12 Months Ended
Dec. 31, 2020
shares
$ / shares
Dec. 31, 2019
shares
$ / shares
Share-Based Payment Arrangements [Abstract]    
Number of stock options outstanding, beginning of period (in shares) | shares 200,631 198,221
Number of stock options granted (in shares) | shares 15,440 7,410
Number of stock options exercised (in shares) | shares (5,900) (2,700)
Number of stock options cancelled (in shares) | shares (5,600) (2,300)
Number of stock options expired (in shares) | shares (31,320)  
Number of stock options outstanding, end of period (in shares) | shares 173,251 200,631
Weighted average exercise price of stock options outstanding, beginning of period (in usd per option) | $ / shares $ 49.07 $ 48.55
Weighted average exercise price of stock options granted (in usd per option) | $ / shares 29.27 57.60
Weighted average exercise price (in usd per option) | $ / shares 34.59 31.73
Weighted average exercise price of stock options cancelled (in usd per option) | $ / shares 58.96 52.31
Weighted average exercise price of stock options expired (in usd per option) | $ / shares 38.24  
Weighted average exercise price of stock options outstanding, end of period (in usd per option) | $ / shares $ 49.44 $ 49.07